1. Field of the Invention
The present invention relates to an image forming method and an image forming apparatus, both adopting an ink-jet recording technique.
2. Description of the Related Art
A so-called ink-jet printer expelling an ink such as a liquid, molten solid or the like from an ink expelling egress such as a nozzle, a slit, a porous film or the like has been well employed as a commercially available printer since it can be down-sized and is not expensive and for other reasons. Known among ink-jet printers are a piezo ink-jet technique in which an ink is expelled using deformation of a piezoelectric element, a thermal ink-jet technique in which a boiling phenomenon of an ink by heat energy is used, or the like technique for reasons of a high resolution, a high speed printability and the like. The above ink-jet printers have been used in printing not only on so-called paper such as plain paper, ink-jet only paper and the like, but also on a film such as an OHP sheet, a cloth or the like as a recording medium.
Now, high speed printing is demanded in an ink-jet printing technique. As one of means effective for realizing high speed printing, there can be named bidirectional collective printing in which printing is performed in forward and return transits of a carriage motion to print pixels in a single scan. In the bidirectional collective printing technique, the printing order of inks is reversed during each scan; therefore, color unevenness caused by the printing order occurs that a hue alters during each scan in an image formed by superimposing plural colored inks one on another. Though this mechanism has not become clear, it is thought that the unevenness originates from a penetrating property of an ink into paper, overlapping between inks or the like.
On the other hand, a method has been proposed in which a treatment liquid (a liquid or a reactive liquid) other than an ink is used for the purpose to improve image quality, better print dryability and the like. For example, a method is publicly disclosed in Japanese Patent No. 2667401 and the like, in which after a liquid including a compound having a cationic group therein is deposited on a recording medium, the liquid then penetrates into the recording medium and stays therein and immediately after the liquid deposit disappears on a surface of the medium, an ink including an anionic dye is deposited onto the surface to form an image. This purports to earn not only dryability, water resistance and light resistance of a printed image, but also a high image quality concerning resolution, clearness, sharpness, image density and the like and further, to improve reliability of a printer such as prevention of nozzle clogging.
Furthermore, a method is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 11-115303 and the like, in which an ink-jet recording technique is adopted in which a reactive liquid is deposited on a recording medium after an ink composition is applied thereon and an interval between coating of the ink composition and the reactive liquid is 5 ms or more and 500 ms or less. This purports to ensure an image having an improved print quality and an increased line width.
In a case of forming an image having at least a secondary color (in the present application, the phrase xe2x80x9chaving at least a secondary colorxe2x80x9d denotes having at least one of a secondary color and tertiary color) at high speed in a technique using inks and a liquid other than the inks or a reactive liquid (a treatment liquid), it has been found that color unevenness and solid color density unevenness is seen due to the printing order of the inks and the treatment liquid. While the mechanism of the unevenness deterioration has not become clear yet, the mechanism is thought to be as follows.
That is to say, in the case where printing performed by printing the treatment liquid, followed by printing at least two colors of ink, colorants coheres and the cohered colorants are stacked in a layered structure according to the printing order. Thus, it is thought that the reason for the unevenness deterioration is because the influence of the hue of the ink, which is printed last, is strong for the hue of secondary colors. In another case where printing is performed with at least two colors of ink, followed by printing of the treatment liquid, if inks with high penetrating property are used, the inks penetrate into the recording medium when printing is performed with the treatment liquid because of high speed penetration of the inks. Therefore, a sufficient effect from a reaction between the inks and the treatment liquid cannot be sufficiently obtained, reducing an improvement to image quality. In addition, color unevenness caused by the printing order also occurs. On the other hand, when inks with a low penetrating property are employed, a surface tension of the inks is large, causing the inks to form a liquid droplet on the recording medium. Thus, portions, at which ink is present, and portions, at which ink is absent, are formed. Therefore, it is supposed that solid color density unevenness tends to worsen.
In a conventional method, in the case where at least two colors of ink and a liquid or a reactive liquid are used, increased image quality, such as improved color unevenness, optical density, bleeding, and inter-color breeding, and higher speed, such as improved drying time, cannot be accomplished simultaneously.
The present invention is intended to solve the above problems in the prior art and achieves the following object as a task. That is, an object of the invention is to provide an image forming method excellent in reduction of color unevenness, optical density unevenness, bleeding, inter-color bleeding, a drying time and the like, caused by the printing order; and an image forming apparatus excellent in applicability to higher speeds.
In order to solve the above task, a serious study has been conducted and as a result, it has been found that in order to simultaneously achieve reduction in color unevenness, optical density unevenness, bleeding, inter-color bleeding, a drying time and the like, caused by the printing order, the printing order of the ink and a liquid composition having a function of cohering the ink, and a penetrating property of the inks into paper are very important, thus achieving the invention.
According to the first aspect the invention, the invention is an image forming method comprising the steps of:
printing at least two colors of ink onto a recording medium using an ink-set, which satisfies the below conditions (i) to (iv) and includes the at least two colors of ink and a liquid composition including an effect of cohering the at least two colors of ink; and
forming an image, which includes a pattern of at least a secondary color, by printing the liquid composition on the printed ink, wherein
(i) an average value of rates of change over time in contact angle of the respective inks on plain paper is at least 1.25 degrees/sec and no more than 3.5 degrees/sec,
(ii) a rate of change over time for a contact angle of each ink on plain paper is less than 4.5 degrees/sec,
(iii) a rate of change over time for a contact angle of a mixture of each ink and the liquid composition on plain paper is in the range of from 5 to 10 degrees/sec and
(iv) the number of coarse particles, which have a diameter of at least 5 xcexcm, in the mixture is at least 1xc3x97104 particles/xcexcL.
According to the second aspect of the invention, the invention is an image forming method, wherein a surface tension of each of the inks is at least 25 mN/m and no more than 40 mN/m.
According to the third aspect of the invention, the invention is an image forming method, wherein a viscosity of each of the inks is at least 1.5 mPaxc2x7s and no more than 6.0 mPaxc2x7s.
According to another aspect of the invention, the invention is an image forming method, wherein a recording head reciprocates along a direction intersecting with a conveyance direction of the recording medium to form an image by adhering the at least two colors of ink on the recording medium, and prints in both forward transit and return transit with the at least two colors of ink, and then prints the liquid composition on the ink printed on the recording medium to form the image including a pattern of at least a secondary color.
According to another aspect of the invention, the invention is an image forming method, wherein an image is formed with a thermal ink-jet technique.
According to another aspect of the invention, the invention is an image forming apparatus comprising:
an ink set, which satisfies the below conditions (i) to (iv) and includes at least two colors of ink and a liquid composition including an effect of cohering the at least two colors of ink;
an ink recording head for printing at least one color of ink, by discharging the at least one color of ink onto a recording medium;
a plurality of liquid composition recording heads for printing the liquid composition on the printed ink, wherein the liquid composition heads are disposed at both ends of the ink recording head in a main scanning direction of the ink recording head, and
(i) an average value of rates of change over time in contact angle of the respective inks on plain paper is at least 1.25 degrees/sec and no more than 3.5 degrees/sec,
(ii) a rate of change over time for a contact angle of each ink on plain paper is less than 4.5 degrees/sec,
(iii) a rate of change over time for a contact angle of a mixture of each ink and the liquid composition on plain paper is in the range of from 5 to 10 degrees/sec and
(iv) the number of coarse particles, which have a diameter of at least 5 xcexcm, in the mixture is at least 1xc3x97104 particles/xcexcL.
According to another aspect of the invention, the invention is an image forming apparatus, wherein a surface tension of each of the inks is at least 25 mN/m and no more than 40 mN/m.
According to another aspect of the invention, the invention is an image forming apparatus, wherein a viscosity of each of the inks is at least 1.5 mPaxc2x7s and no more than 6.0 mPaxc2x7s.
According to another aspect of the invention, the invention is an image forming apparatus, wherein the image forming apparatus reciprocates along a direction intersecting with a conveyance direction of the recording medium to form an image by adhering the at least two colors of ink on the recording medium, and prints in both forward transit and return transit, then prints the liquid composition on the ink printed on the recording medium to form an image including a pattern of at least a secondary color.
According to another aspect of the invention, the invention is an image forming apparatus, wherein the image is formed with a thermal ink-jet technique.
According to another aspect of the invention, the invention is a set of inks and liquid composition, wherein the inks satisfy the below conditions (i) to (iv) and includes the at least two colors of ink and the liquid composition including an effect of cohering the at least two colors of ink,
(i) an average value of rates of change over time in contact angle of the respective inks on plain paper is at least 1.25 degrees/sec and no more than 3.5 degrees/sec,
(ii) a rate of change over time for a contact angle of each ink on plain paper is less than 4.5 degrees/sec,
(iii) a rate of change over time for a contact angle of a mixture of each ink and the liquid composition on plain paper is in the range of from 5 to 10 degrees/sec and
(iv) the number of coarse particles, which have a diameter of at least 5 xcexcm, in the mixture is at least 1xc3x97104 particles/xcexcL.
According to another aspect of the invention, the invention is a set of inks and a liquid composition, wherein a surface tension of each of the inks is at least 25 mN/m and no more than 40 mN/m.
According to another aspect of the invention, the invention is a set of inks and a liquid composition according to claim 11, wherein a viscosity of each of the inks is at least 1.5 mPaxc2x7s and no more than 6.0 mPaxc2x7s.